Named after Frederick Sanger, a British chemist who used the compound as part of a procedure to determine the amino acid sequence of bovine insulin. This eventually led to the synthesis of artificial insulin, and a Nobel Prize for Sanger. Sanger's Reagent is commonly used in analytical chemistry to form derivatives of compounds that do not have a strong chromophore, allowing them to be more easily detected and measured. Because this reagent has a benzene ring and two nitro groups, it strongly absorbs ultraviolet light due to massive delocalization of electrons throughout the molecule. Sanger's Reagent is most commonly used to derivatize amino acids, but can be useful for just about any compound with an amine group. Because the fluorine and nitro groups withdraw electron density from the benzene ring, a carbon atom in the ring undergoes nucleophilic attack by the N-terminal nitrogen of an amine group, forming a compound that will strongly absorb light in the UV and/or visible spectrum.

Sources:MSDS for 2,4-dinitrofluorobenzene for physical data
www.xrefer.com for info on Sanger
I'm a chemist, I actually remember some of this stuff.